bsd-user: elf cleanup

Move OS-dependent defines into target_os_elf.h. Move the architectural
dependent stuff into target_arch_elf.h. Adjust elfload.c to use
target_create_elf_tables instead of create_elf_tables.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Kyle Evans <kevans@FreeBSD.org>
Signed-off-by: Justin Hibbits <chmeeedalf@gmail.com>
Signed-off-by: Alexander Kabaev <kan@FreeBSD.ORG>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>